Air carrier flight crew reported while on approach they received a TCAS RA which resulted in descent below glide path and a low altitude alert from ATC.
Synopsis
Air carrier flight crew reported while on approach they received a TCAS RA which resulted in descent below glide path and a low altitude alert from ATC.
Narrative
After diverting twice and holding for 30 minutes prior to arrival at SFO; given clearance for the visual approach 28R noticed traffic descending on TCAS at our 5 o clock could not see them revived a descending RA (aircraft configured gear down flap 5 on approach speed) disconnected autopilot (AP) auto throttles (AT) pitched to the fly to zone; notified ATC. They asked our intention decided to continue since we could not climb. Around 1200 afe monitor vertical speed and visual contact with the airplane causing the RA occurred. Started to level off; received glide slope caution (backing up the visual approach with the ILS 28R) said correcting saw 1 white three red on the PAPI received one more glide slope caution responded correcting still seeing 1 white three red on the papi got to 2 white 2 red; the rest of approach was in uneventful.Faster Aircraft Y paired behind slower Aircraft X caused an RA. Because we responded and Aircraft Y did not we had to continue our descent lower then if after we said we were responding to an RA. If Aircraft Y were to have climbed instead of continuing their approach then the RA conflict would have been resolved in a more appropriate way. Once RA traffic insight and monitor vertical speed (v/s) speed occurred was moderately task saturated and flying a stabilized approach; while looking for traffic caused my scan lapse and to go momentarily lower than glide slope.Suggestions: Unless both aircraft respond to an RA when one aircraft says it's responding then it makes it hard for a successful outcome. By continuing the approach Aircraft Y caused us to continue to go lower with no ability to go around until around 1200agl. Even though the approach met stabilized criteria if I could have done it again I would go around once Aircraft Y was in sight and deemed not a factor anymore.
Second reporter narrative
On our second diversion of the day; we were given a visual approach clearance for 28R into SFO. We could see the traffic on TCAS behind us; but we did not have the traffic in sight when we got the RA. The Captain disconnected autopilot and auto throttles and complied with the RA until we had the faster traffic in sight (it was a Aircraft Y on approach to the parallel runway). The First Officer advised ATC that we were complying with the RA and that we would continue the approach since we were responding to the RA by descending. ATC gave an altitude alert at the same time the plane gave a glide slope caution and the Captain was already correcting the altitude when both warnings came. We were fully configured at about 1200 AFE when this all occurred and we were stable when the First Officer gave the 1000 ft call. We continued the approach to land.A faster Aircraft Y paired behind a slower Aircraft X caused an RA. Because the Aircraft Y had us in sight and did not respond to the RA we were forced to descend on an approach and momentarily dip below the glide slope causing the altitude alert from ATC and glide slope caution until we had that aircraft insight. The Captain was extra task saturated in those few seconds that passed because the Aircraft Y was on their side of our plane; so they had to fly and look for traffic.Suggestions: ATC should be aware of approach speeds and not have faster traffic behind and overtake the slower traffic unless both aircraft have each other in sight. Or both aircraft should respond to RA so that if the one ahead doesn't have the one behind them in sight (and is forced to descend as the response) they would not descent to an unsafe altitude or situation. We also could have gone around at the first traffic warning (before the RA) to avoid the RA.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.